The present invention relates to a fluorine-containing resin composition comprising a fluorine-containing prepolymer containing a carbon-carbon double bond in its molecular chain, a fluorine-containing nonlinear optical material obtained by curing the composition and an optical waveguide device obtained from the fluorine-containing nonlinear optical material.
A nonlinear optical material of the present invention means a starting substance which exhibits a nonlinear optical effect when used on functional optical devices and optical parts. The nonlinear optical effect means an optical effect arising as a result of a nonlinear polarization becoming not ignorable in polarization of atom and molecule which is induced by strong external electric field. For example, Pockels effect, parametric oscillation, second harmonic generation, etc. are known in a second-order nonlinear optical effect, and Kerr effect, electrostatic induction SHG, third harmonic generation (THG), change in a refractive index due to optical intensity, etc. are known in a third-order nonlinear optical effect. The second-order nonlinear optical effect can be applied on a wavelength conversion (SHG) device, electro-optical modulator, etc. and the third-order nonlinear optical effect can be applied on a wavelength conversion (THG) device, optical computing by high speed optical shutter, optical bistability device, optical switching, etc.
Inorganic ferroelectric materials such as potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO3) and the like have been so far known as nonlinear optical materials. However those materials have a problem that a response speed is slow due to deliquescence, low dielectric breakdown strength and further high dielectric constant, and therefore an applicable frequency range is limited. Also 2-methyl-4-nitroaniline (MNA) providing a higher nonlinear optical response than that of the above-mentioned inorganic ferroelectric materials was found by B. F. Levine, et al. in 1979, and since then research and development of organic nonlinear optical materials have been proceeded positively.
With respect to those organic nonlinear optical materials, applications thereof as new optical materials can be expected since a larger nonlinear optical constant, a higher nonlinear optical response and a higher dielectric breakdown strength can be expected as compared with inorganic ferroelectric materials. Further the organic nonlinear optical materials have an advantage that a wide variety of molecular design conditions and applications on various devices can be considered, and therefore attention is directed thereto as basic materials in the field of opto-electronics in the future.
Actually bulk single crystals obtained from MNA, 1-(4-nitrophenyl)-3,5-dimethylpyrazole (DMNP), (2,2-dicyanovinyl)anisol (DIVA) and 4′-nitrobenzylidene-3-acetoamino-4-methoxyaniline (MNBA) are used as a wavelength conversion material. Those materials are processed into fiber type or slab type optical waveguides for trial manufacture of optical frequency multiplier devices (Optical Functional Material, pp. 41-105, Kyoritsu Shuppan).
However when using the bulk single crystals, productivity and processability are inferior and therefore the use thereof is not suitable for mass production. On the other hand, in case of optical waveguides produced using high molecular weight materials, a film forming process by a spin coat method, etc. can be employed and therefore production process is easy and an area of the waveguide can also be made large. Further generally the high molecular weight materials are free from deliquescence and are high in a dielectric breakdown strength and therefore are superior to inorganic materials. There have been studied many trials to use, as a nonlinear optical material, a high molecular weight composition comprising a high molecular weight material containing an organic nonlinear material.
Such a high molecular weight composition generally has no orientation and cannot be used as it is as a material for optical switches and modulation devices which make use of an electro-optical effect. Generally there is employed a method of applying, for orientation, a direct current field on a high molecular weight material free from orientation while heating, namely a method of exhibiting an electro-optical effect by poling treatment.
However there is a serious problem that after the poling, when the material temperature is decreased to normal temperature and then is allowed to stand or is used, orientation thereof is lost and an electro-optical effect disappears. So far energies have been concentrated on a study of polymethacrylate (PMMA), etc. as a high molecular weight material for an optical waveguide, but a glass transition temperature (TG) thereof is as low as about 150° C. and there is a problem that the orientation generated by the poling disappears completely when heat history of not less than 200° C. is unavoidably suffered during production of an optical integrated circuit.
In order to exhibit a nonlinear optical effect efficiently in the optical waveguides, it is desired that a material to be used has a large nonlinear optical constant and is excellent in light transmission properties (transparency). When light source having a wavelength of 1.3 μm or 1.5 μm band is used for those optical waveguides, since a carbon-hydrogen bond and oxygen-hydrogen bond of the organic material have absorption of light having a wavelength in the above-mentioned range, there arises a phenomenon that light transmission is decreased and a nonlinear optical effect is lowered. Accordingly studies have been made to replace such hydrogen atom with heavy hydrogen (D) or with fluorine. As a result, though lowering of transparency can be improved to a certain extent, in case of the replacement with heavy hydrogen, water absorbing properties of the material does not change but absorption of near infrared light is increased due to absorption of water and transparency is lowered. In case of the replacement with fluorine, increase in a glass transition temperature is not recognized and the above-mentioned problem (disappearing of orientation) is not solved.
As a high molecular weight material which has a good transparency in a near infrared wavelength region, is relatively low in loss of light and has a low water absorption, there are proposed non-crystalline fluorine-containing perfluoro polymers having a ring structure (JP3-9329A, JP4-190202A, JP2000-81519A, etc.).
Though those non-crystalline fluorine-containing polymers have no problem with transparency, there is the above-mentioned problem (disappearing of orientation) attributable to poor heat resistance, that is, a low glass transition temperature. In case of a polymer system having a sufficiently increased glass transition temperature by changing a structure and components thereof, the polymer itself becomes fragile and there is a problem that cracking arises in a process for forming a waveguide. Thus all the problems in nonlinear optical materials have not been solved.